1. Field of the Invention
The present invention relates to an antenna device, and in particular to an antenna device having no less than two antenna elements.
2. Description of the Related Art
There is a trend that radio apparatus like mobile phones are equipped with not only so called a cellular-type mobile communication system but also various kinds of radio systems such as a wireless local area network (WLAN), the global positioning system (GPS), a radio identification system (RFID), a terrestrial digital television system and so on. It is anticipated that the above trend continues to grow, and that the radio apparatus will increasingly have multiple uses and multiple functions.
In response to advances in multiple uses and multiple functions of the radio apparatus, features of multiple resonance and broader frequency ranges are increasingly required of antenna devices provided in the radio apparatus. Meanwhile, downsizing and compactness are also required of the antenna devices from a viewpoint of improvement of designs and downsizing of the radio apparatus. In order to meet the above requirements conflicting to each other, the radio apparatus has to be equipped with an antenna device adapted for plural radio systems.
Configuration of such an antenna device may be divided broadly into two types of approaches. One of the approaches is a configuration where plural antenna elements (maybe including a parasitic element) having different resonant frequencies one another are combined, commonly fed by and distributed to plural systems via an antenna sharing device such as a switch or a duplexer.
Another one of the approaches is a configuration where plural antennas are arranged close to each other in a space-efficient manner, and each of the antennas is separately fed by an associated system.
In the configuration using the antenna sharing device, an isolation characteristic of the antenna sharing device dominates isolation among the systems different one another. In order to compensate for shortage of the isolation of the antenna sharing device, another device such as a band-pass filter may be needed. Consequently, increased insertion losses of the antenna sharing device and the filter may cause basic performance such as transmitter power or receiver sensitivity to be degraded.
The configuration where each of plural antennas is separately fed by an associated system is advantageous to basic performance of radio apparatus, as there is no need to think of insertion losses of antenna sharing devices and filters. Meanwhile, there is a problem that isolation may hardly be assured as the antennas are arranged spatially close one another.
For such a problem, conventional antenna devices have been proposed so that isolation may be assured, as disclosed in Japanese Patent Publication of Unexamined Application (Kokai), No. 2003-332840 and No. 2005-198245.
More specifically, the invention of the antenna device disclosed in JP 2003-332840 was applied by the applicant of this application so as to reduce a cross coupling between antennas of an antenna device arranged on a same grounded conductive plate and to reduce leakage of electromagnetic waves from a transmitting antenna to a receiving antenna. The antenna device is provided with a plate-like short-circuiting element on and almost perpendicular to the grounded conductive plate between feed portions of the antennas, and configured to block views between the feed portions for solving the above problems.
The antenna device disclosed in JP 2005-198245 is configured that one of two antenna elements is a half wavelength long with a grounded end so as to work equivalently to a loop antenna of a wavelength long and to suppress resonance occurring on a ground plane. It is mentioned that even if another one of the antenna elements is excited at a nearby frequency, a coupling between the antenna elements may be suppressed as antenna current distribution is small near a feed portion of the equivalent loop antenna.
The antenna device disclosed in JP 2003-332840 by the applicant of this application is configured to have the plate-like short-circuiting element perpendicular to the grounded conductive plate so as to isolate between the antennas. Such a configuration may not be very suitable for a small sized radio apparatus such as a mobile phone which is required to be small and thin.
The antenna device disclosed in JP 2005-198245 is on an assumption that frequency bands of use of the antenna elements are close to each other. It is thus restricted to apply such a configuration to broad multiple uses and multiple functions of radio apparatus.